1. Field of the Invention
The present invention is generally directed to the field of hydraulic actuators, and more particularly to a system for controlling a hydraulic actuator, and various methods of using same. In one illustrative example, the present invention is directed to a system for controlling an actuator for a downhole safety valve in a subsea Christmas tree.
2. Description of the Related Art
The production from a subsea well is controlled by a number of valves that are assembled into a Christmas tree. The designs of actuators and valves for subsea wells are dictated by stringent safety and reliability standards, because of the danger of uncontrolled release of hydrocarbons. These valves have traditionally been powered by hydraulic fluid. However, it has recently been proposed to use electrically powered actuators instead, as these offer many advantages. In such subsea systems, all the low-pressure hydraulic actuators are replaced with electrically powered actuators, thus eliminating the entire low-pressure hydraulic system.
Many countries have a requirement for a downhole safety valve (Surface Controlled Subsurface Safety Valve, SCSSV) as an additional safety device for closing the flow path in the well tubing. Since this valve is located remote from the other valves it has its own dedicated actuator. Normally a hydraulic actuator is used, and because the valve is located in the tubing, and thereby in the pressure stream, it must be operated by high-pressure hydraulic fluid. This fluid supply is normally transmitted through a separate line from a special high-pressure supply.
It would be desirable eliminate the high-pressure hydraulic system as well. One possibility that has been contemplated is to omit the SCSSV from the system, thus eliminating the need for high-pressure hydraulic power. However, since SCSSV's are required equipment in many locations they cannot be omitted from all systems. Also, because of the harsh downhole environment, it is accepted as not being reliable to replace the hydraulic SCSSV actuators with less robust electric actuators. Although the high-pressure hydraulic system remains necessary, it would still be desirable to reduce the number and/or complexity of the components that make up the high-pressure system.
To avoid the costs of a dedicated high pressure line from topside several alternatives have been proposed, such as an electrically powered pump, a pressure intensifier, or an accumulator that stores high pressure fluid subsea. These alternatives, however, are complicated, making them generally less reliable and more costly than traditional systems. Also, these alternatives require that more equipment be deployed subsea than in a traditional system.
The present invention is directed to an apparatus for solving, or at least reducing the effects of, some or all of the aforementioned problems.